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Exocytosis: Definition & Examples

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  • 0:01 What Is Exocytosis?
  • 1:45 Regulated Exocytosis
  • 3:03 Constitutive Exocytosis
  • 3:36 Lesson Summary
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Lesson Transcript
Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Like a factory, cells make products. Some of these stay in the cell and others are exported. This lesson is about how cellular products are shipped out of the cell using a process called exocytosis.

What Is Exocytosis?

Exocytosis is a process by which a cell transports secretory products through the cytoplasm to the plasma membrane. Secretory products are packaged into transport vesicles (membrane-bound spheres). Let's look at some examples of cellular secretory products:

  • Secreted protein - enzymes, peptide hormones, and antibodies
  • Neurotransmitters from nerve cells
  • Plasma membrane proteins
  • Antigens - pieces of bacteria or other invaders which stimulate the immune response

Exocytosis can be either calcium-dependent or calcium-independent. In calcium-dependent exocytosis, an influx of calcium into the cell will stimulate secretion. This happens when a protein is only secreted when needed. Transport vesicles with their cargo will travel from the Golgi vesicle (the protein-packaging organelle) to the plasma membrane. The vesicle then docks at the plasma membrane and waits for a secretion signal. This is also called regulated exocytosis because secretion from the vesicles is controlled.

In contrast, calcium-independent exocytosis occurs constantly. Just like with regulated secretion, the transport vesicle and its cellular product will travel through the cell to the plasma membrane. Unlike regulated secretion, the vesicle does not wait at the plasma membrane. Instead, fusion with the plasma membrane and secretion of vesicle contents into the extracellular environment happens automatically. This constant flow of secretory product is also called constitutive exocytosis because secretion from the vesicles is not controlled. The following figure illustrates the differences:

Differences in Exocytosis

Example: Regulated Exocytosis

Cells will use the products they secrete to communicate with each other. An example of this is a neuron, or nerve cell. How does this work? Say you want to move your big toe. Neurons in your brain are in contact with neurons in your spine. The neurons in your spine are in contact with neurons that lead to your big toe. In your big toe, neurons are in contact with the muscles in your big toe.

For you to move your big toe, your thought has to be turned into an electrical impulse. This impulse will travel down your nerves to your toe muscles. This stimulates your toe muscles to contract, and your toe moves! So, how does exocytosis come into play? Well, there is a gap between each neuron. The gap is very tiny, but large enough to stop the electrical impulse.

Luckily, your neurons store neurotransmitters (signal molecules) at the plasma membrane in this gap. The electrical impulse travels down the neuron to the gap. At the gap, the electrical impulse stimulates neurotransmitter release. The cell on the other side of the gap will bind to the neurotransmitters, and an electrical impulse begins in the recipient cell. This continues until the impulse reaches its destination: the muscles of your big toe. The following figure illustrates this point:

Regulated Exocytosis

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